Add docs about character sets and charset switch

[ibarwick]

Hi

Thanks for the PR. I've taken the opportunity to look deeper into Firebird's handling of character set names and aliases; the Firebird source code provides a canonical reference (see: https://github.com/FirebirdSQL/firebird/blob/master/src/jrd/IntlManager.cpp#L100 ) which supports my original assertion that there is quite a good overlap between PostgreSQL character set names and those supported by Firebird. There is a small subset where no mapping exists; I have identified these as:

• SQL_ASCII → NONE
• ISO_8859_5 → ISO8859_5
• ISO_8859_6 → ISO8859_6
• ISO_8859_7 → ISO8859_7
• ISO_8859_8 → ISO8859_8
• WIN866 → DOS866
• EUC_JP → EUCJ_0208 (with caveats)

and commited an update supporting these (96e1625).

Note that PostgreSQL supports some character sets (e.g. SJIS) as client encodings, but not as server character sets, so there's no need to consider those at all.

I'll update the documentation separately.

Thanks once again.

[mkgrgis]

Thank for additional edit, @ibarwick ! I look forward to when these changes will be used by you in part or in whole for the main branch.

[ibarwick]

And documentation changes added with 9dd98ff (visible mainly here and here ).

Thanks for the suggestions once again.

[mkgrgis]

I'm glad my suggestions were partially helpful, @ibarwick !